One of the formidable problems in multilevel metallization of integrated circuits is obtaining continuity of the second level metal over the step formed by the dielectric coated first level metal pattern. The problem arises from the difficulty in maintaining an adequately thick metal layer as it traverses the step. Moreover, during the metal deposition process voids tend to develop at a strip step edge. These voids trap etchants and other processing fluids causing corrosion and contamination of the device.
These problems are well known in the integrated circuit industry. To avoid them it is desirable to deposit the metal layer in such a manner that shallow bevel angles result at any step edge that is subsequently to be covered.
It can be mentioned at the outset that these metallization patterns are typically formed of aluminum. This description will deal primarily with aluminum metallization although it will be recognized that the teachings described may as effectively be applied to other metallurgical systems. It is known that when aluminum is deposited on silicon, typically for contacts, the silicon is soluble in aluminum sufficiently to pit the silicon. This is unacceptable in many cases, for example, if the device has shallow junctions. An Al-Si alloy (1-2% Si) is used to avoid this problem. The ensuing description is as applicable to the alloy as to aluminum. It is also applicable to polysilicon metallization.
Processing parameters and reliability of two level aluminum metallization structures have been investigated by P. B. Ghate, et al., IEEE Trans. on Reliability, Vol. R22, 186 (1973). Their technique to bevel the aluminum is to standardize photoresist operations and optimize the etch temperature. No quantitative description of the bevel angle is given. T. Agatsuma et al., Meeting of the Electrochemical Society, May 1973, have recently reported an aluminum beveling procedure which utilizes a differential etch rate technique on anodized aluminum. Bevel angle obtained by this differential etching varied from 6.degree. to 45.degree..